The present invention relates to a high frequency amplifier for the UHF range from 300 MHz to 3 GHz with an output power in the megawatt range.
More particularly, the invention relates to a high frequency amplifier of the type including a device for generating an electron beam, the device comprising an input resonator provided with an input gap in front of which a cathode ring is disposed. In such a device means are provided for generating a high frequency electrical field in the input resonator for stimulating the cathode to produce a rotating electron beam. A grid ring is provided at the side of the gap opposite the cathode ring so that the electron beam passes through the grid and is accelerated in a direct voltage acceleration path. The grid prevents the passage of the electrical field from the input resonator into the acceleration path. An output resonator is positioned to receive the beam and to convert the kinetic energy of the electrons, which enter the output resonator with approximately identical velocities, into high frequency electrical power in dependence on the azimuthal angle width of the beam.
High frequency amplifiers operating in a frequency range above 200 MHz to several GHz with a power in the megawatt range are gaining increasing significance in relation to high current accelerators, as for example, the spallation neutron source and the accelerator breeder, and for use in fusion technology. The efficiency of such a power amplifier is of special interest, particularly because of rising energy costs.
A klystron is disclosed in the Handbuch der Elektronik (Electronics Handbook) published by Franzis-Verlag, Munich, 1st edition, 1979, pages 426-429, which is a UHF power amplifier in which the velocity of the electrons of an electron beam is modulated by the high frequency electric field of a control resonator. In a drift path downstream of the control resonator, electrons travelling at different velocities can catch up with one another and form electron bunches. In an output resonator, the density modulated electron beam is decelerated and its kinetic energy is converted to high frequency power. However, because the electron bunches travel, for structural and electrical reasons, at bunching angles lying in a range between 90.degree. and 180.degree., only part of the electrons can be decelerated in an optimum manner and converted to HF power in the output resonator. The total high frequency efficiency of the klystron thus lies at about 65%.
For that reason, an amplification principle has been developed which produces the HF excitation of the output resonator by modulation of the azimuthal entrance angle width of the electron beam entering into the output resonator rather than by density modulation of the electron beam as in the klystron. A UHF power amplifier operating according to this principle is known as a radial gyrocon and is disclosed in the IEEE Transactions on Electron Devices, volume ED-26, No. 10, October 1979, pages 1559 to 1566. An electron beam of high velocity generated by an electron gun is deflected out of the axis of the gyrocon by a deflection resonator in which a rotating wave is generated, and is additionally deflected by a magnetic dipole so that the beam enters into an annular output resonator. The azimuthal angle width of the electron beam in the gyrocon is only about 60.degree. and the velocity of all electrons is approximately equal so that almost completely deceleration of the electrons is possible. With suitable selection of the parameters a total HF efficiency of 80% can be realized. The known radial gyrocon, however, is very large, structurally complicated and requires additional electrical power for the magnetic dipole.
A UHF power amplifier known as a trirotron, which stands for triode producing a rotating beam for RF amplification, is disclosed in Stanford Linear Accelerator Center Publication 2266, presented at the 1979 Particle Accelerator Conference, San Francisco, California, March 12-14, 1979, and constitutes a further development of the gyrocon. The trirotron described therein is circular and includes a cathode ring disposed at the inside of the input gap of a cylindrical input resonator. During every negative halfwave of the high frequency, the cathode emits electrons which exit radially relative to the axis of the input resonator through a grid disposed opposite the cathode at the exterior wall of the resonator into a direct voltage acceleration path. In a likewise cylindrical output resonator which is coaxial with the input resonator, the accelerated electrons are slowed down and their kinetic energy is converted into HF power. The smaller the azimuthal angle width of the electron beam, the greater is the efficiency of the conversion, which may reach approximately 85% at a 60.degree. angle. The smaller azimuthal angle width is realized by a direct electrical field which is superposed over the HF electrical field of the input resonator. The total efficiency of the known trirotron lies at 80%.